You can turn it off, but would you. Would you risk turning it off at work where everyone else that looks at your doc sees the purple and you never even got a heads up.
If Google were to affect search ranking for something like that, it would be much simpler to just affect it based on the presence of the actual keywords that would have been underlined than retaining some metadata about the configuration of the editor when the document was written.
Spoiler alert: Google already has a deny list of words that will cause a page to not show up outside of focused queries for it. Most people have no idea how much porn is actually on the internet.